1. The role of specific retroviral polytropic envelope sequences in induction of brain disease has been studied in a mouse retroviral model. By infection of neural stem cells with these retroviruses disease induction was enhanced and the pathogenic potential of new envelope gene sequences was revealed. Comparison of two related viral clones has identified two different envelope regions which are important to disease induction. One region appears to influence the amount of viral protein produced and the other appears to influence the toxicity of the protein in infected brain cells. 2. Direct measurement of interferon-gamma in cultures of Friend virus-specific T lymphocytes identified the importance of certain helper T cell subsets in the process of recovery from Friend virus leukemia.3. The mouse Rfv-3 gene, which influences the ability of Friend virus infected mice to make neutralizing antibodies to Friend virus, has been mapped in more detail to a specific 0.8cMregion on chromosome 15. This location is nearby, but distinct from Ly6 and Wnt7b genes but could not be separated from Il2rb, Il3rb and Pdgfb genes.